Serious accidents and death have occurred by reason of premature detonation of explosive well tools such as perforating guns, casing or tubing cutters, string shot rods and jet cutters. These devices include powerful explosives that are extremely dangerous, and must therefore be handled with great care. As discussed in my application Ser. No. 483,355, filed Feb. 22, 1990, now U.S. Pat. No. 4,967,048, issued Oct. 30, 1990, explosive tools can fire accidentally due to various causes. The different switch embodiments disclosed in such patent prevent the arming of the tool until after the explosives have been positioned below the rig floor, so as to prevent accidental firing while the tool is being rigged up at the surface. While these switches represent significant advances in the art, there also is a great need for a switch that not only will prevent premature detonation at the surface, but also will prevent such detonation unless the explosive well tool is below a predetermined depth in the well bore. Thus not only is the tool disarmed until it has been lowered beyond such depth, the tool is automatically disarmed at such depth it is being withdrawn from the well, so that there is no danger of discharge of explosives at the surface that failed, for some reason, to fire in the well.
The general object of the present invention is to provide a new and improved bypass safety switch that will prevent firing of an explosive well tool until the tool and switch are submerged beyond a certain depth in a fluid-filled well bore.
Another object of the present invention is to provide a new and improved safety sub that operates to prevent flow of enough electric current to the detonator fuse of an explosive well tool unless the sub is being subjected to a predetermined hydrostatic pressure in a well bore.